[1] [2] [3]	[1] This first image is a low power longitudinal section through an onion Allium cepa root tip. We are showing it upside down: the thimble shaped region at the top would normally be the deepest point into the earth in its natural environment. The cap detects gravity and controls the direction of root growth. [2] You can see the root cap in better detail here. Notice the dense region of cells behind it called the apical meristem. This is the site of major cell division where the cells differentiate (change) into specialised cells to form the central cylinder; the cylinder contains xylem and phloem. Around the central cylinder is the cortex. The zone behind the apical meristem is known as the zone of elongation. It is a region which has xylem cells making up annular rings, or spiral secondary cell walls, so they can elongate as well as transport water. [3] This last high power image of the apical meristem shows the cells very clearly. They are very active as can be seen by the large size of their nuclei relative to the size of the cells. Two of the cells in the central region can be seen dividing by mitosis; the chromosomes are very visible since the two cells are in their early telophase of the cell cycle.
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